The present invention relates to a semiconductor device and, in particular, to a device comprising Darlington-connected drive stage and output stage transistors.
A typical arrangement of a Darlington-connected transistor circuit is illustrated in FIG. 2 as including a drive stage transistor 1 coupled by its emitter to the base of an output stage transistor 2. To increase the speed of the switching operation of this circuit, a diode 3 is coupled from the base of transistor 2 to the base of transistor 1.
The switching operation of the Darlington-connected circuit of FIG. 2 is shown diagrammatically in FIG. 3, in which I.sub.C represents the output or collector current and I.sub.B represents the input or base current. The circuit of FIG. 2 has a turn-off time t defined by the period from cutoff of base current I.sub.B to the point at which output current I.sub.C becomes 0.
To improve the performance of the Darlington-connected transistor circuit, it is desirable to increase its switching speed, i.e., to decrease the duration of turn-off time t. Prior attempts to achieve this result have had negative effects on other aspects of the circuit, however. For example, where the Darlington-connected transistor circuit is formed in a single semiconductor substrate, reducing the lifetime in that substrate has affected both driver and output stage transistors 1,2 so that the current amplification factor h.sub.FE deteriorates. This drawback is particularly problematic when the semiconductor substrate being utilized is of a relatively large size.